Molded plastic containers, more specifically, containers molded from expanded organic polymer material, are available in different shapes and sizes for packaging a variety of fragile articles, such as eggs, light bulbs and the like. Such containers are particularly useful to protect the fragile articles from impact, changes in temperature and other adverse environmental effects.
The technique for forming such containers, well known in the industry, involves heating a wide, continuous sheet of plastic material to soften it for molding. The softened sheet is then advanced repeatedly in between the upper and lower portions of multi-cavity molds and the mold portions are pressed together against the softened sheet to form the containers. When pressed together, the mold portions are cooled to fix the plastic in its molded shape. The mold portions are then separated and the process is repeated a number of times.
A majority of the containers have a cover that latches over a base portion of the container. Such containers utilize latching arrangements which must withstand rough and rapid handling, or repeated opening and closing. A simple yet effective latching arrangement is obtained by providing a latch, extending outwardly from or adjacent to one edge of either the cover or base portion, which is insertable into an opening provided in the other portion. The latch can be a flat tab, a round protuberance or the like.
In early methods, latch openings were formed in a separate punching process after the cartons were molded. This separate process required additional equipment and labor, thereby increasing the cost of manufacturing such containers.
Subsequent methods, such as described in U.S. Pat. No. 3,845,187, eliminate the separate punching process. The latch opening is formed during the molding process itself. As the mold portions close, the softened sheet of plastic is stretched over a die surface on one of the mold portions, and a hole former carried by the other mold portion is pressed into contact with the die surface to cut through the plastic along one edge of the latch opening. As the mold portions close further, resilient means are used to cause the hole former to scrape along the die surface and push the plastic across a scraped region, away from the cut, to form a compressed wall of plastic spaced from the cut to form the latch opening. However, the requirement that the hole former be in contact with the die surface, when it cuts through the plastic sheet, necessitates periodic sharpening or replacement of its dulled or blunted cutting edge.
Another method utilizes a heavy spring top-loaded onto a rotary shear key to obliquely bias the key surface and resist the horizontal vector of the downward travel of the hole former upon this surface. Again, this results in a considerable scraping action between the hole former and the key surface and misalignment of the key.